Descriptions of flywheel batteries and their various related elements can be found in U.S. Pat. Nos. 5,614,777 set forth by Bitterly et al; 567,595, 5,708,312, 5,770,909, and 58,644,303 by Rosen et al; 3,860,300 and 4,147,396 by Lyman; 3,791,704 and 4,088,379 by Perper; 5,627,419 by Miller; 4,910,449 by Hiyama et al: 5,760,510 by Nomura et al: 5,777,414 by Conrad; 5,319,844 by Huang et al; 4,444,444 by Benedetti et al; 5,844,339 by Schroeder et al; 5,495,221, 5,783,885, 5,847,480, 5,861,690, and 5,883,499 by Post; 5,705,902 by Merritt et al; 5,044,944 and 5,311,092 by Fisher; 5,107,151 and 5,677,605 by Cambier et al; and 5,670,838 by Everton; plus 3,969,005, 3,989,324, 4,094,560, and 4,141,607 by Traut; and 4,799,809 by Kuroiwa.
This invention relates to electric power storage with minimal losses, through power interface electronics and electromechanical energy conversion, in the inertia of a spinning flywheel, and by reciprocal means, stored kinetic energy conversion to electric power. The various component elements of the invention include: A high-speed motor/generator, with cooperative power electronics and magnetic bearings, electronic feedback control servos to stabilize the magnetic bearings, a vertical-axis flywheel, integral with the motor/generator rotor and rotatable magnetic bearing elements, to store kinetic energy, a vacuum enclosure to reduce air drag, mechanical backup bearings that are not engaged during normal service, and a stationary energy-absorbing installation site to safely house the flywheel and its enclosure.
Rechargeable electrochemical batteries are commonly used for storing on-site and portable electric power. Lead-acid types provide the highest energy-to-cost and power-to-cost ratios. But they require frequent maintenance, fail without warning, are very heavy, corrosive, deteriorate over time, consist of mostly hazardous materials with disposal problems, and their lifetimes are limited to under ten years xe2x80x94far shorter if subjected to repeated charge/discharge cycles or not promptly recharged after supplying power. These battery drawbacks have been a major obstacle to on-site solar and wind power installations. To provide power on demand, such installations require power storage that is subjected to daily and highly variable charge/discharge cycles.
Flywheel power storage devices, and the various elements needed for their implementation, have been set forth in the prior art, with various combinations of elements developed as alternatives to electrochemical batteries. The term xe2x80x9cflywheel batteryxe2x80x9d has been used for these combinations of elements related to the prior art, and is used herein by way of reference to the complete power storage and recovery system of this invention.
Flywheel batteries of the prior art can supply only short-term power, and their energy is typically dissipated in less than an hour, due to high idling losses. This energy loss, without supplying output power, is far worse than self-discharge exhibited by electrochemical batteries.
Prior art magnetic bearings, for use in flywheel batteries, that employ thermal, hysteresis, and eddy effects, for moving mechanical devices, to adjust physical positions of magnetic materials for stability control, confront serious stability and reliability problems. Others, using superconductor techniques (conductor repulsion of moving magnets), confront high idling losses.
Magnetic bearings have also been described for flywheels that operate in a weightless space environment. They include, at opposing ends of a flywheel spindle, permanent magnets, and electronic servos to adjust magnetic forces for virtually zero power axial positioning and stabilization. Their design is not suitable for use in a terrestrial environment, as they require opposing magnets far too large and expensive for commercially viable flywheel batteries.
Typical prior art motor/generators, used in flywheel batteries, have efficiency of about 90%, with a substantial part of their loss in core laminations subjected to high frequency pulse width modulation. Moreover, idling loss due to iron cores magnetically cycled by permanent-magnet spinning rotors, causes high self-discharge rates. With such high power conversion and idling loss, excessive heat is generated within the evacuated flywheel enclosure. This heat can cause a variety of failure modes. It also can cause excessive maintenance requirements, which prevent practical and safe installation, of flywheel batteries intended for stationary on-site use.
In the prior art, methods to reduce idling losses have included means for separating the rotor and stator during periods when no power is converted. These methods confront greatly complicated mechanical structures, and inability to quickly respond when the flywheel battery system needs to supply power. As with motor/generators based upon induced field machines (e.g., alternating current induction motors), or various machines having variably excited field windings, when field excitation is removed to reduce idling loss, they cannot perform as generators unless connected to an external power source, such as electrochemical batteries.
A type of motor/generator, known in the art as coreless (in that its stator windings are not in core slots), has also been used in prior art flywheel batteries. They incur high eddy current loss in their stator windings. Those with stepwise commutation also incur rotor hysteresis and eddy loss, when converting power. Rotor heat does not have a conductive path to the enclosure, in systems having contactless magnetic bearings, so high internal temperatures may be incurred. These losses, incurred while converting power and when idling, have heretofore not been adequately investigated and explained, and have been mistakenly attributed to skin effect.
Explosion hazard of spinning flywheels is a serious concern. Safety measures, that depend on the flywheel""s vacuum enclosure to contain a possible exploding rim, add significant cost and weight, and are not always effective. Almost all fiber-composite rim flywheel batteries spin at rim surface speeds above Mach 1. They are contained within evacuated enclosures, to prevent high windage losses. Maintenance needs of prior art flywheels preclude the safe, low cost siting and installation methods set forth in this invention.
In the prior art, idling loss has been largely due to friction in mechanical bearings, and to motor/generators and magnetic bearings that magnetically cycle iron as the rotor spins, causing substantial hysteresis and eddy losses. The prior art also includes many combinations of magnetic bearings that are stabilized and assisted by mechanical bearings of various types. Some use a motor/generator having standard mechanical bearings, coupled to a flywheel by materials having radial compliance to minimize vibration stresses.
Mechanical bearings of the prior art incur serious heating and wear, running in vacuum at sustained high speed. Very high operating temperatures of critical parts, has been caused by high localized heat generation compounded by low heat transfer, further compounded by lubrication loss accelerated by lubricant boil-off in vacuum. These conditions have resulted in early mechanical bearing wear, deterioration, and high failure rates.
Vacuum loss in the prior art necessitates relatively frequent maintenance to keep windage loss at acceptably low levels. High temperatures cause lubricants to boil and some composite fiber flywheel resins to outgas into a relatively small evacuated enclosed space. In the prior art, the enclosed space has been small, to minimize size and weight of the enclosure, which has thick walls designed to contain a possible exploding flywheel rim. A small enclosure space, with high internal temperatures and materials that outgas, does not reliably maintain a vacuum.
At high temperature, even coercive force of permanent magnets is reduced. This has required more adjustment of magnetic bearings, imposed higher loads on mechanical bearings, and caused reduced torque vs. current of motor/generators, with reduced back-emf vs. rotational speed.
Flywheel power storage systems that are not subject to the aforementioned drawbacks and limitations would afford significant improvement to numerous useful applications. These include uninterruptible power supplies (UPS) to sustain critical electric power functions (during power outages), in buildings commonly served by a central power grid, and UPS plus power storage for on-site distributed solar and wind power systems.
Therefore, general objectives of this invention are to provide improved long-life flywheel batteries, for stationary installations, without the power losses, internal heating, vacuum loss, maintenance, explosion hazard, high cost, and malfunctions of the prior art; plus improvements to related elements needed for the flywheel batteries.
A primary and specific objective is to provide a flywheel battery having a motor/generator with no magnetic hysteresis and minimal eddy current losses, and with mechanical bearings used only as temporary backup; its rotor integral with magnetic bearings that need virtually no steady-state power to drive their axial and radial position stabilizing electromagnets, that also do not incur magnetic hysteresis and eddy current losses, and that can use low-cost magnets.
Another specific objective of this invention is to provide electromechanical power conversion at greater efficiency, with less idling loss, by the flywheel battery motor/generator. This also minimizes stator winding heating, resulting thermal stresses, and possible outgassing of resin molding, and provides maximum overall energy storage efficiency.
Another objective of this invention is to especially minimize losses and consequent heat in all flywheel parts which spin in the vacuum enclosure without physical contact and thermal conduction paths to any stationary parts that can transfer heat out of the enclosure, to minimize thermal stresses and composite fiber flywheel resin outgassing.
Another objective is to eliminate need for lubricants in mechanical backup bearings, to remove a cause of vacuum loss, frequent maintenance, and mechanical bearing failures.
Another objective is to maintain all electronic components, and critical regions therein, at cool temperatures, with minimal temperature cycling; also to provide low cost rotor angle and axial and radial position sensors and circuits with good signal integrity that is substantially free of ground loops, extraneous pickup, and cross-talk and further to provide over-current and over-voltage protection for the electronic components.
Another objective, consistent with and dependent upon the previous objectives, is to provide a flywheel battery that does not require maintenance over a very long service lifetime; to reduce maintenance cost and especially to reduce need for access to the flywheel and its vacuum enclosure, so as to facilitate housing the flywheel in a site capable of absorbing any possible explosive blast, in the possible event of a flywheel rim disintegration, to protect people in and around the building served, and prevent damage to the building, from a flywheel explosion.
Another objective of this invention is to detect possible flywheel rim deterioration or seismic shaking, by resulting flywheel axial and radial vibration, and begin a controlled flywheel slowdown and spindle capture by backup mechanical bearings, to minimize flywheel damage and prevent explosion, from possible rim disintegration or electronic control malfunction.
Yet another objective, consistent with and dependent upon the previous objectives, is to produce a low cost and low weight vacuum enclosure for the flywheel, with more interior vacuum space than in the prior art, to maintain vacuum longer, without maintenance, and to enable safe flywheel operation, installed and housed in a stationary site as described herein.